I have a 2009 Silverado with 26k miles. When I pull up to a stop and sit for awhile I get a clunk sound and it feels as if someone has hit me from behind. I'm guessing it is coming from the transmission--maybe the torque converter down shifting?? Does anyone else have this problem, if so-what did you do to correct it?? Sometimes it happens right away and sometimes it happens a few minutes after sitting for a red light. Any help out there?? Thanks!